Embodiments of the present invention relate generally to control systems, such as those used in magnetic storage systems and methods and, in specific embodiments, to systems and methods that correct for disturbances to coarse actuator control caused by external vibrations using feed-forward techniques that are selectively enabled or disabled based on track follow loop signals.
Magnetic storage systems, such as disk drives, are widely used in computers and other electronic devices for the storage and retrieval of data. Important design considerations for disk drive manufacturers generally include: (a) data storage capacity; (b) data transfer rate; (c) data integrity and reliability; and (d) manufacturing cost.
In general, related art disk drives comprise one or more disks for storing data, an actuator arm, and one or more transducers or heads. Each head is operable to read data from and write data to concentric circular tracks on a surface of a corresponding disk. The heads are typically attached to the actuator arm, and when a head performs a read or a write operation, the actuator arm is moved so that the head is positioned over the center of a selected track to perform the desired operation.
In recent years, disk drive manufacturers have sought to increase the data storage capacity of disk drives while controlling the manufacturing cost. One solution has been to increase track density by increasing the number of tracks per inch (TPI) on each disk. As TPI has increased, tracks have become narrower, and maintaining data integrity has become a greater design challenge because data errors can occur with smaller amounts of movement of a head away from a track center during a read or a write operation.
Movement of a head away from a track center can lead to an off-track read or an off-track write. An off-track read occurs when a head is positioned over a wrong track during a read operation and the head reads data from the wrong track. In such an instance, the incorrect data would have to be discarded, the head repositioned over the correct track, and the head would then have to read in the correct data. As a consequence, the data transfer rate of the disk drive would be reduced, because the time spent reading the wrong data would be wasted. Even worse than an off-track read is an off-track write. An off-track write occurs when a head is positioned over a wrong track during a write operation and the head writes data to the wrong track. As a result of an off-track write, data integrity is adversely affected, because existing data on the wrong track is improperly overwritten and is potentially lost.
Thus, to prevent data errors, it is preferable to maintain a head over a center of a selected track during a read or a write operation. In order to position a head during a read or a write operation, related art disk drives typically comprise a servo controller and have embedded servo sectors located in the tracks of each disk. The embedded servo sectors are located between data sectors and contain predetermined patterns from which a position of a head during an operation can be determined.
During read and write operations to a selected track, a head reads data from embedded servo sectors of the selected track and provides the data read from the embedded servo sectors as servo information to a servo controller. The servo controller receives the servo information provided by the head and determines a position error signal (PES) from the servo information. The PES is indicative of the position of a head relative to the center of the selected track. The PES is then fed into a compensator that produces an appropriate compensation signal so that the actuator arm will reposition closer to the center of the selected track. Once the actuator arm is repositioned, the process repeats as the head again reads data from the embedded servo sectors and provides updated positional information to the servo controller. This interplay between the PES, compensator, and the positional information regarding the actual location of the head, form the track follow loop.
When operating in various environments, a disk drive may be subject to various external forces in the form of vibrations or shocks. Depending on the intensity and direction of these external forces, the actuator arm and head assembly can become displaced from their desired location over the center of a track. Translational forces will not have a significant impact on the position of the head if the actuator arm assembly is balanced. However, rotary forces acting in the plane of the disk may cause considerable head displacement. Although convergence of the track follow loop does provide some protection against such displacement by eventually re-positioning the head over the center of the track, the loop has a finite response time that might be too slow to correct for certain vibratory or other forces acting on the disk drive.
It has been proposed to use accelerometers to sense disruptive forces acting on a disk drive. The accelerometers generate signals representative of the intensity and direction of the forces acting on a disk drive, and these signals in turn can be used in a feed-forward architecture to make the disk drive more robust to such forces.
Various types of accelerometers, for example linear accelerometers and angular accelerometers, have been used in such compensatory schemes. Linear accelerometers detect forces acting in one direction (translational forces), whereas angular accelerometers detect rotational forces acting within some plane. As mentioned before, translational forces are not a particular threat to balanced actuator arm assemblies, and thus angular accelerometers are more useful. However, linear accelerometers may also be used in pairs to detect rotational force. The signal difference between two linear accelerometers affixed at opposite ends of a disk drive will yield a value close to zero in the presence of translational force since both accelerometers will notice acceleration in the same direction. However, in the presence of rotational force, each accelerometer will notice acceleration equal and opposite of the other since at any given moment they will be accelerating in opposite directions. Thus, in the presence of rotational force, the absolute value of the signal difference will constructively add.
Forces acting normal to the plane of rotation (z-axis) of disks of a disk drive are not a particular source of concern because the actuator arm assembly and head will not be displaced in a direction along the plane of disk rotation (x-y axes). Therefore, rotational accelerometers or linear accelerometers will be positioned such that their directions of sensitivity are parallel to the plane of the disk. Otherwise, the correctional information these accelerometers provide will contain components pertaining to disturbances parallel to the z-axis—information that may mislead the acceleration feed-forward system in trying to correct for a disturbance that is actually not affecting the position of the head.
Ideally, an accelerometer that is situated so as to only detect motion in the x-y axis will not generate a signal in response to forces being imparted on it from the z-axis. In reality however, an accelerometer situated so as to only detect motion in the x-y axis may still generate non-zero signal information in response to forces directed from the z-axis. In response to such signal information, an acceleration feed-forward system may move the head in order to compensate for what it erroneously thinks to be a threatening disturbance within the plane of the disk. In this way the acceleration feed-forward system can become a source of noise itself, and make it more difficult for the head to converge onto the center of the selected track. In extreme situations, this noise can cause off-track read or write errors, which is unacceptable.